public class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
  }

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || root == p || root == q) return root;
        TreeNode l = lowestCommonAncestor(root.left,p,q);
        TreeNode r = lowestCommonAncestor(root.right,p,q);
        if (l == null) return r;
        if (r == null) return l;
        return root;
    }
}